(1) Field of the Invention
The present invention relates to a converting controller. More particularly, the invention relates to a converting controller for setting an over current protection value and a time period by the same pin.
(2) Description of the Prior Art
FIG. 1 is a schematic circuit of a conventional buck converter. Referring to FIG. 1, the buck converter is adapted to convert an input voltage Vin into a stable output voltage Vout, which includes a controller 10, a transistor switch NM, a diode D, an inductance L, an output capacitance Co, and an output voltage detector VD. The output voltage detector VD detects the output voltage Vout to generate a voltage feedback signal Vfb. The controller 10 is a constant on time controller and determines whether to turn on the transistor switch NM according to the voltage feedback signal Vfb. When the output voltage Vout is lower than a predetermined voltage, the controller 10 turns on the transistor switch NM for a constant on time, such that the power from the input voltage Vin is transmitted to the inductance L and the output capacitance Co through the transistor switch NM. After the constant on time, the transistor switch NM is turned off, and then the current through the diode D flows to the inductance L, so as to release the energy stored in the inductance L to the output capacitance Co. In order to avoid the current flowing through the transistor switch NM is over high to damage the transistor switch NM or the inductance L, the controller 10 receives a current detecting signal Cse representing the current value flowing through the transistor switch NM, wherein the current detecting signal Cse is generated by the on-state resistance of the transistor switch NM. When the current flowing through the transistor switch NM is larger than an over current protection value, the controller 10 immediately turns off the transistor switch NM in the cycle period.
The controller 10 includes a feedback comparator 11, an on-time setting circuit 12, a driving logic judgment circuit 14, an over current comparator 15, and an over current setting circuit 16. The on-time setting circuit 12 is outside coupled to an on-time setting resistance Rton to set the on-time period through a pin of the controller 10. The over current setting circuit 16 is outside coupled to an over current setting resistance Rocs to set an over current protection reference signal OCREF through another pin of the controller 10. When the voltage feedback signal Vfb is lower than a reference voltage signal Vre, i.e., the output voltage Vout is lower than the predetermined voltage, the feedback comparator 11 outputs a feedback comparing signal Com to the on-time setting circuit 12. Then, the on-time setting circuit 12 generates an on-time signal Ton with constant pulse width, which is set by the on-time setting resistance Rton. When the driving logic judgment circuit 14 receives the on-time signal Ton, the transistor switch NM is turned on, such that the power from the input voltage Vin is transmitted to the output capacitance Co to raise the output voltage Cout. When the current detecting signal Cse is lower than the over current protection reference signal OCREF, the voltage drop crossing the transistor switch NM operates is over high due to the current flowing through the transistor switch NM being larger than the over current protection value. Then, the over current comparator 15 outputs an over current protection signal OCP to the driving logic judgment circuit 14, such that the driving logic judgment circuit 14 turns off the transistor switch NM to avoid the problem of over current. When the output voltage Vout is lower than the predetermined voltage next time, the driving logic judgment circuit 14 turns on the transistor switch NM again.
The controller 10 sets the over current protection value and the on-time period through the different pins to be applied to the different applied situation. However, the above method increases the number of the pins of the controller 10 and so causes higher cost.